1. Field of the Invention
The present invention relates to a PM stepping motor, and more particularly to a terminal structure of a PM stepping motor.
2. Description of the Related Art
A PM (permanent magnet) stepping motor (hereinafter referred to simply as “stepping motor”) includes a rotor assembly including a circular cylindrical magnet having a plurality of magnetic poles disposed circumferentially, and a stator assembly including two stator units axially coupled to each other and disposed around the rotor assembly. Each of the two stator units includes a pair of yokes (inner and outer yokes) which each include a circular ring shaped base portion and a plurality of pole teeth disposed along the inner circumference of the base portion, bent upright and arranged in a comb-like manner, and which are disposed to oppose each other such that respective plurality of pole teeth mesh with each other, wherein a coil for exciting the pair of yokes is disposed around the intermeshing pole teeth (refer to, for example, Japanese Patent Application Laid-Open No. 2008-136302).
The exciting coil is structured usually such that a magnet wire is wound on a cylindrical bobbin disposed around the intermeshing pole teeth. Both ends (starting and finishing ends) of the wound magnet wire (winding) are wrapped around pin-like terminals implanted into, for example, a terminal block which is formed integrally with the bobbin so as to be located over the base portion of the inner yoke and a flange of the bobbin facing the base portion. The ends of the coil winding wrapped around the terminals as described above are soldered for ensuring electrical connection thereto. When an electrical signal from a control circuit disposed externally is applied via the terminals to the exciting coil structured as described above, each of the pole teeth of the pair of yokes is excited, whereby attraction force or repulsion force is generated between each pole tooth and the magnetic pole of the magnet thereby causing the magnet to rotate by a predetermined angle.
The ends of the coil winding may be reliably connected electrically to the terminals by, for example, plasma welding (fusion processing) rather than by using a soldering method. By using a welding method such as plasma welding, the use of solder is eliminated thus enhancing the safety and the environmental sustainability (refer to, for example, Japanese Patent Application Laid-Open No. 2001-286090).
FIG. 7 shows a terminal 81 adapted for connection by plasma welding disclosed in Japanese Patent Application Laid-Open No. 2001-286090, which has a biforked shape as a whole, specifically structured to integrally includes an internal connection portion 82 having an L-shape and having a magnet wire connected thereto, an external connection portion 83 connected to an external circuit, and a root portion 84 force-inserted into a terminal block 87 formed at a part of a flange 86 of a bobbin 85, wherein the external connection portion 83 and the root portion 84 are connected to each other so as to form a straight bar and one bar of the L-shaped internal connection portion 82 is orthogonally connected to the connection part between the external connection portion 83 and the root portion 84.
After one wire end of a coil winding formed on the bobbin 85 is wrapped around the internal connection portion 82 of the terminal 81 structured as described above, when a discharge current is applied to the internal connection portion 82 with the external connection portion 83 functioning as a grounding electrode, the one wire end of the winding can be plasma-welded to the terminal 81.
In a stepping motor disclosed in Japanese Patent Application Laid-Open No. 2001-286090, one pair (two) of terminals, to which, respectively, both wire ends of a coil winding to constitute one stator unit are connected, are arranged along a direction orthogonal to the rotation axis (defined orthogonal to the plane of the paper In FIG. 7). Also, the pair of terminals are disposed at a terminal block such that an internal connection portion having a magnet wire wrapped therearound and an external connection portion connected to an external circuit are aligned in a virtual plane (corresponding to “terminal alignment direction” to be described later) orthogonal to the rotation axis. Another pair (two) of terminals for another stator unit, which are arranged in the same manner as the one pair of terminals described above, are disposed to axially oppose the one pair of terminals (refer to FIG. 3 of Japanese Patent Application Laid-Open No. 2001-286090). That is to say, on the whole of the stepping motor, four terminals are arranged at a face of the terminal block in two arrays such that two terminals are provided in each array.
The number of motor steps must be increased in some cases for the purpose of increasing torque and enhancing resolution in rotor advance angle of a stepping motor. In order to increase the number of motor steps, it is required to increase the number of magnetic poles of a magnet and the number of pole teeth of yokes, but if the number of pole teeth is increased while maintaining the inner diameter of a stator, the width (circumferential dimension) of each pole tooth is inevitably reduced.
Under the circumstances described above, if the yokes are to be produced by punching, the thickness of a material plate to be punched must be reduced in consideration of the punching process limitation in order to reduce the width of the pole tooth or reduce a gap between adjacent pole teeth to thereby allow the number of pole teeth to be increased. Consequently, the thickness of a circular ring shaped base portion of the yoke must also be reduced, and the axial direction dimension of a terminal block formed at the outer circumference of the base portion of the yoke (inner yoke) is accordingly reduced, which results in making it difficult for the terminals to be arranged in two arrays in the axial direction. In order to cope with the difficulty, the thickness of a flange of a bobbin may be increased as one solution, but this hinders reduction of the stepping motor in the axial direction and does not necessarily make an acceptable solution.